This application is related to the art of wireless communications and more particularly to transmission for wide-band wireless systems.
In a wireless communications system, a given base station will generally be established to serve a number of mobile communications stations located within the serving area of that base station. Each such base station is assigned a set of communications channels developed from a portion of the radio frequency spectrum assigned to the particular wireless service provided via the base stationxe2x80x94which assignment function may be dynamic. For each of such channels, the function of transmitting information intended to be received by a designated mobile station is generally distributed among four transmission components: (1) a set of electronics carrying out the function of modulating the communications signal onto a carrier signal (hereafter referred to as a xe2x80x9cradioxe2x80x9d); (2) an RF power amplifier for increasing the power of radio frequency signals, such amplifier being designed to avoid adding significant distortion products; (3) a transmit filter for attenuating residual harmonics and other RF emissions outside the desired transmit frequency; and (4) a transmission antenna.
Because of various constraintsxe2x80x94e.g., cost, esthetic, zoning, etc.xe2x80x94it is often considered desirable to minimize the number of antennas for the transmission of multiple communications channels served by a given base station. In large systems where these multiple communications channels are transmitted using more than one RF signal (or carrier), such an arrangement necessarily requires that the RF signals representing those multiple channels be combined prior to reaching the antennas serving those channels. Various arrangements are known in the art for accomplishing such combining, and the combining equipment selected will have a significant effect on the design of the RF amplifiers within the system. Two of the more commonly used methods may be described as Individual Carrier Linear Amplifier (ICLA) systems and Multiple Carrier Linear Amplifier (MCLA) systems. Salient characteristics, and differences, for each of those systems are described briefly below.
An exemplary (and somewhat simplified) ICLA system is schematically illustrated in FIG. 1. As can be seen from the figure, each radio 101 has it""s own dedicated amplifier 102, with the outputs of the amplifiers being combined in a broadband RF combiner 103 and sent to the transmit filter 104 and antenna 105. The combiner must provide sufficient port-to-port isolation to prevent generation of spurious products due to leakage of signals between ports of the combiner and subsequent mixing of the signals in the amplifiers. When each ICLA amplifier is designed to meet all in-band spurious emission requirements (within the service provider""s licensed transmit band), and assuming sufficient port-to-port isolation in the combiner, only one transmit band filter is required at the output of the combiner to limit out-of-band emissions. This provides flexibility since carriers (channels) can be added or moved without the need to change or add filters. The price of this flexibility is high, however, since the overall system efficiency with this arrangement is poor in systems with large numbers of carriers. The reason for the poor system efficiency is that the individual RF carriers are combined in a broadband combiner at high power levels after amplification. Broadband RF power combiners are very inefficient when combining multiple signals at different frequencies. In fact, the signal loss from the input to the output of a typical broadband combiner is proportional to the number of signals being combined. For an N-to-1 combiner, the power loss per signal is equal to (Nxe2x88x921)/N.
For the 4-to-1 combiner shown in FIG. 1, three-fourths of the output of each amplifier is dissipated in the combiner, which leaves only one-fourth of the amplifier output power for the antenna. Broadband combined ICLA configurations are therefore best suited for systems operating at low power levels and/or with low numbers of carriers.
In order to combine the outputs of ICLA amplifiers without excessive losses, it is common practice to frequency isolate their outputs by passing them through individual bandpass filters prior to combining. This narrowband combining prevents the high losses which would otherwise occur in a system using broadband RF combiners. Note, however, that for a given filter selectivity, the passbands of these filters must be sufficiently separated in frequency in order to effectively reduce the combining losses. Since the individual filters are tuned to specific frequencies it is difficult to re-configure or grow such a system without extensive filter replacement and re-cabling. Although remotely tunable combiners are available, these are generally more complicated and expensive than fixed-tuned combiners, with complexity and cost increasing as the passband signal bandwidth increases.
An exemplary MCLA system is schematically illustrated in FIG. 2. As can be seen in that figure, with the MCLA approach, the outputs of the individual radios 201 are combined in a broadband RF combiner 202 prior to amplification. This multi-carrier signal is then amplified by the MCLA amplifier 203 and passed through a transmit filter 204 prior to being sent to the antenna 205.
The MCLA amplifiers are designed to meet all spurious emission requirements within the service provider""s licensed transmit band, and therefore, only one transmit band filter is required to limit out-of-band emissions. This provides additional flexibility since carriers (channels) can be added or moved without the need to change or add filters. The price of this flexibility is high, however, since amplifiers of this type are more complicated and expensive than amplifiers which only process one RF carrier.
One of the important characteristics of an MCLA amplifier is that it is capable of processing a large number of RF signals simultaneously while still maintaining low intermodulation distortion. Thus, an MCLA amplifier provides more flexibility than the ICLA approach since any number of carriers may be amplified by a single amplifier, as long as the cumulative output power remains below the average output power rating of the amplifier. Also, since the carriers are combined at low power levels prior to amplification, power loss due to combining signals at different frequencies is reduced. However, the MCLA amplifier is typically less efficient than its ICLA counterpart, since it must operate further below the saturation level than the ICLA amplifiers in order to maintain intermodulation distortion products at an acceptably low level. MCLA amplifiers also typically employ feed-forward linearization circuitry to further reduce intermodulation distortion, which increases the size and cost of the MCLA amplifier.
An object of the invention is accordingly a low cost RF amplifier architecture for multi-carrier, wide-band wireless applications. To that end a methodology is disclosed for processing carriers in a wireless application whereby the outputs of a multiplicity of modulated RF signal sources are combined into a single RF signal stream, that single RF signal stream being thereafter divided among the inputs of another multiplicity of RF power amplifiers. The outputs of that multiplicity of RF power amplifiers are then recombined, filtered and provided as an input to a transmission antenna. According to the method of the invention, the RF amplifiers are selected to meet a set of predetermined spurious emission requirements and are established in a cooperative relationship with the transmit filter in order to provide amplified output signals meeting in-band spurious emission requirements.